(core) Add rules to eslint to better match our coding conventions.

Summary:
We used tslint earlier, and on switching to eslint, some rules were not
transfered. This moves more rules over, for consistent conventions or helpful
warnings.

- Name private members with a leading underscore.
- Prefer interface over a type alias.
- Use consistent spacing around ':' in type annotations.
- Use consistent spacing around braces of code blocks.
- Use semicolons consistently at the ends of statements.
- Use braces around even one-liner blocks, like conditionals and loops.
- Warn about shadowed variables.

Test Plan: Fixed all new warnings. Should be no behavior changes in code.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2831

app/client/models/rowset.ts

@@ -368,25 +368,25 @@ export class FilteredRowSource extends BaseFilteredRowSource {
  * Private helper object that maintains a set of rows for a particular group.
  */
 class RowGroupHelper<Value> extends RowSource {
-  private rows: Set<RowId> = new Set();
+  private _rows: Set<RowId> = new Set();
   constructor(public readonly groupValue: Value) {
     super();
   }
 
   public getAllRows() {
-    return this.rows.values();
+    return this._rows.values();
   }
 
   public getNumRows(): number {
-    return this.rows.size;
+    return this._rows.size;
   }
 
   public _addAll(rows: RowList) {
-    for (const r of rows) { this.rows.add(r); }
+    for (const r of rows) { this._rows.add(r); }
   }
 
   public _removeAll(rows: RowList) {
-    for (const r of rows) { this.rows.delete(r); }
+    for (const r of rows) { this._rows.delete(r); }
   }
 }